Historical biogeography supports Point Conception as the site of turnover between temperate East Pacific ichthyofaunas

ABSTRACT The northern and southern marine faunas of the temperate Northeastern Pacific meet within California as part of one of the few eastern boundary upwelling ecosystems of the world. Traditionally, it is believed that Point Conception is the precise site of turnover between these two faunas due to sharp changes in oceanographic conditions. However, evidence from intraspecific phylogeography and species range terminals do not support this view, finding stronger biogeographic breaks elsewhere along the coast. Here I develop a new application of historical biogeographic approaches to uncover sites of transition between faunas without needing an a priori hypothesis of where these occur. I used this approach to determine whether the point of transition between northern and southern temperate faunas occurs at Point Conception or elsewhere within California. I also take advantage of expert-vetted latitudinal range data of California fish species from the 1970s and the 2020s to assess how biogeography could change with the backdrop of climate change. The site of turnover was found to occur near Point Conception, in concordance with the traditional view. I suggest that recent species- and population-level processes such as dispersal and speciation could be expected to give signals of different events from historical biogeography, possibly explaining the discrepancy across studies. Species richness of California has increased since the 1970s, mostly due to new species from Baja California. Range shifts under warming conditions will have the consequence of increasing the disparity between northern and southern faunas of California, creating a more divergent biogeography.